The present invention relates to a agents useful as excitatory amino acid antagonists. The quinoline and kynurenin derivatives of the present invention have affinity for the glycine binding site of the NMDA receptor. Compounds of the present invention should be usable for the treatment of epilepsy, treatment of neurodegenerative diseases, prevention of eschemic/hypoxic damage to cerebral tissue or other syndromes involving inhibition or excessive stimulation of the NMDA receptor complex.
There is an increasing body of evidence that excitatory amino acid neurotransmitter systems are progressively affected in the course of Alzheimer's disease (AD) (Greenamyre, et al.; Foster et al., Penney et al.). Excitatory amino acid neurotransmitter systems are important in memory and learning (Izumi et al.; Morris et al.), and have been implicated in a number of CNS disorders including epilepsy (Wong et al.), hypoxia/ischemia brain damage (Rothman), Huntington's disease (Young et al.), AIDS encephalopathy (Giulian et al.), and amyotrophic lateral sclerosis (Spencer et al.). A specific set of these excitatory receptors are of particular interest, those that selectively bind N-methyl-D-aspartic acid (NMDA). Activation of this transmitter system is probably a necessary early step in the formation of certain types of memory (Collingridge et al.), yet over-stimulation of these receptors can be toxic and result in cell death (Rondouin et al.). It should be feasible to design and synthesize efficacious amino acid analogs as therapeutic agents for this receptor, once binding characteristics are understood. However, it needs to be remembered that a direct agonist for the NMDA receptor could be neurotoxic, while an antagonist might potentiate memory loss. Research relating to the present application has been directed towards the glycine modulatory site on the NMDA receptor complex (Johnson et al.). Development of a strategy for pharmacological intervention at this site should lead to the production of drugs with mixed agonist-antagonist activities such that they are clinically useful while having minimal side effects.
The NMDA-sensitive receptor sites comprise a subset of the excitatory neuroreceptors that are activated by L-glutamic acid. This particular type of receptor is coupled to an ion channel which is voltage dependent and permeable to calcium. The receptor complex also has a strychnine-insensitive binding site for glycine. Occupation of this glycine site is thought necessary to produce channel opening. There is evidence that NMDA receptors are responsible, at least in part, for the neurotoxicity seen in ischemia, and to excitatoxic cell death (Rondouin et al.). Excitatoxicity is the neuronal degeneration caused by exposure of CNS tissue to excitatory amino acids. It has been shown that non-competitive antagonists of NMDA receptors protect cortical and hippocampal cell cultures against glutamate neurotoxicity (Rondouin et al.). Calcium entry through the NMDA receptor channel is thought to be the mechanism by which glutamate released from nerve terminals can regulate long-term physiological events and, under pathological conditions, precipitate neurodegeneration.
As used in the present application: the term "halogen" refers to fluorine, chlorine or bromine; the term "lower alkyl group", refers to a branched or straight-chained alkyl group containing from 1 to 6 carbon atoms; the term "pharmaceutically acceptable addition salt" applies to any nontoxic organic or inorganic addition salt of the basic compounds described herein. These may be acid addition salts or basic addition salts. Typical inorganic acid addition salts include those resulting from hydrochloric, hydrobromic, sulfuric, phosphoric and acid methyl salts such as sodium monohydrogen, orthophosphate and potassium hydrogen sulfate. Illustrative organic acids which form acceptable salts include mono-, di- and tricarboxyllic acids, for example, acetic, glycolic, lactic, pyruvic, malonic, succinic, glutaric, fumaric, malic, tartaric, citric, ascorbic, maleic, hydroxymaleic, benzoic, hydroxybenzoic, phenylacetic, and so forth. Pharmaceutically acceptable basic addition salts applied to any organic or inorganic basic addition salts including those resulting from alkali metal or alkali-earth metal hydroxides, ammonia, aliphatic, alicyclic or aromatic organic amines.
Epilepsy is a complex disease process with many possible etiologies. This is reflected in the variety of chemicals which have been used therapeutically to treat seizures. Yet some 20-40% of epileptic patients fail to experience satisfactory seizure control with the drugs currently available (Goehring et al., 1990). An antiepileptic compound can affect either the initiation of the epileptic discharge or its spread within the brain. In either case, the drug ultimately must attenuate neuronal excitability. This can be accomplished by at least three different mechanisms: modulation of voltage-dependent ion channels, enhancement of CNS inhibitory pathways, or suppression of excitatory pathways (Rogawski, et al. 1990). An ideal drug should act specifically at the site of inappropriate excitation, rather than broadly inhibiting neuronal activity. In this manner, untoward side effects such as ataxia and sedation can be minimized. Pathways utilizing the excitatory neurotransmitter L-glutamic acid offer several potential targets for therapeutic intervention. Since neuroreceptors for this amino acid transmitter exist as several pharmacologically different subtypes (Watkins et al., 1990), therapeutic agents can be selected on the basis of binding specificity for a given receptor subset.
The N-methyl-D-aspartate (NMDA) activated subset of neuroreceptors for L-glutamate is linked to a nonselective cation channel (Nowak et al., 1984). For channel opening to occur, however, a strychnine-insensitive glycine binding site on the receptor-channel complex must be occupied (Johnson and Ascher, 1987). Antagonism of glycine binding at this site inhibits NMDA responses (Kemp et al., 1988). Also located on this receptor-channel complex is a binding site for the dissociative anesthetic phencyclidine. Occupation of this site also blocks NMDA-mediated responses in a noncompetitive manner (Snell and Johnson, 1986). Since glutamate functions as an excitatory neurotransmitter, it is not surprising that competitive inhibitors of NMDA activity have been shown to have anticonvulsant properties (Dingledine et al., 1990). Chemicals which noncompetitively antagonize NMDA activity by binding either at the phencyclidine site (such as MK-801) or at the glycine site (7-chlorokynurenic acid) likewise demonstrate anticonvulsant activity (Wong et al., 1986; Singh, et al., 1990).
The glycine binding site on the NMDA receptor complex presents a unique target for treatment of seizures arising in NMDA activated neurons. Chemical analogs of excitatory amino acid transmitters usually are ionized at physiological pH. Consequently, they are best applied directly into the CNS, as charged molecules often have difficulty crossing the blood-brain barrier. This limits their usefulness as therapeutic agents. Two different groups of aromatic chemicals have been reported to interact at the NMDA-associated glycine site: kynurenic acids (4-hydroxy-2-quinolinecarboxylic acid) (Kemp, et al., 1988) and indole-2-carboxylic acids (Huettner, 1989). Derivatives lipophilic enough to pass from the systemic circulation into the CNS can be produced from each of these groups. Through the use of ligands for the glycine site, NMDA receptor activity can be modulated without interfering with other excitatory transmission pathways. Furthermore, binding characteristics of the NMDA-associated glycine sites are distinct from those of the inhibitory (strychnine-sensitive) glycine receptors in the spinal cord, with both kynurenate (kyn) (White, et al, 1989) and indole-2-carboxylic acid (Huettner, 1989) being inactive at the strychnine-sensitive sites.
The present invention involves, in one aspect, a series of kynurenic acid derivatives and indole esters as well as the anticonvulsant potential of these compounds and their potencies as glycine inhibitors. Also tested were pyridine-2,6-dicarboxylic acid and its corresponding diethyl ester, based on structural similarity to kynurenic acid and a report by Stone and Perkins (1981) that 2,3-pyridine dicarboxylic acid acts on NMDA neurons. Anticonvulsant evaluation was conducted against both maximal electroshock (MES) and maximal electroshock induced seizures (Swinyard and Kupferberg, 1985; Porter, et al., 1984). Rotorod testing was used to determine if the compounds caused a neurological deficit. A synaptosomal assay was utilized to determine strychnine-insensitive glycine binding inhibition (Jones, et al., 1989).